Japanese Patent Application Laid-Open No. HEI 4-148074 and 4-148076 describes an ignition coil, an ion current detector unit and a switching element integrated into a single unit for all the cylinders of an internal combustion engine so as to achieve a compact construction and reduce the number of component parts required, thereby improving reliability. Japanese Patent Application Laid-Open No. 4-148076 describes a detecting device and a switching device integrated into a single unit for each cylinder of an internal combustion engine so as to achieve a compact construction and reduce the number of component parts required, thereby improving reliability.
In the detection of a misfire in an internal combustion engine, a typical diagnostic system outputs a misfire diagnostic signal and turns on a misfire diagnostic lamp. However, since the misfire diagnostic lamp can be turned on due to various factors, it is difficult to identify the cause of a misfire diagnosis even when the diagnostic lamp is tuned on. For example, when the misfire diagnostic lamp is turned on due to a connector contact failure, the cause is difficult to identify if the connector contact failure is one that has a low reproducibility. In the technology described in Japanese Patent Application Laid-Open No. HEI 4-148074, since various components are integrated into a single misfire detection unit for all the cylinders, it is difficult to determine which cylinder is experiencing the misfire. Therefore, the entire misfire detection unit for all the cylinders needs to be replaced. Moreover, this technology is unable to detect a wire breakage or a connector contact failure. The technology described in Japanese Patent Application Laid-Open No. HEI 4-148076 is able to check component parts separately for each cylinder when a misfire is detected, but is not able to detect a wire breakage or a connector contact failure.